The increasing popularity of portable consumer electronic products such as mobile phones, MP3 players and PDAs has prompted new concerns on the huge variety and number of battery chargers that are required. This number is both inconvenient to users and eventually leads to electronic waste problems. Inductive or wireless charging apparatus that can charge more than one electronic product have been proposed. Two different approaches have been proposed for the ac magnetic flux generation, namely “horizontal flux” and “vertical flux” method.
Inductive electronic chargers have been developed for use with some types of portable electronic equipment such as electric toothbrushes. Inductive chargers have also been proposed in U.S. Pat. Nos. 6,356,049, 6,301,128, and 6,118,249. These inductive type chargers, however, use traditional transformer designs with windings wound around ferrite magnetic cores. The main magnetic flux between the primary (energy-transmitting) winding and secondary (energy-receiving) winding has to go through the magnetic core materials. An alternative contactless charger [Chang-Gyun Kim; Dong-Hyun Seo; Jung-Sik You; Jong-Hu Park; Cho, B. H., “Design of a contactless battery charger for cellular phone,” IEEE Transactions on Industrial Electronics, Volume: 48, Issue: 6, December 2001 Page(s): 1238-1247.] proposed also uses magnetic cores as the main structure for the coupled transformer windings. However, these battery chargers do not use a planar structure and each charger is able to charge only one electronic load at a time.
Two different approaches to planar battery charging devices have recently been proposed. The first type of planar battery charger modifies the rotating machine concept by flattening the “round shape” of the motor into a “pancake shape,” as shown in FIG. 1(a) and reported in GB2399225A, GB2398176A, WO2004/038888A, GB2388716A, US2003-210106A, GB2392024A, and GB2399230A. The magnetic flux lines 1 flow along (i.e., roughly parallel to) the planar charging surface 2. However, such a horizontal flux approach requires a vertical surface to pick up the ac flux for voltage induction (FIG. 1(b)) and this limitation makes it difficult to design a slim energy-receiving module that can be unobtrusively housed inside the equipment to be charged. Typically, as shown in FIG. 1(b) the secondary winding needs to be wound round a magnetic core 3.
The second approach (shown for example in WO03/105308A, GB2389720A, GB2399446A, U.S. Pat. No. 7,164,255, GB2389767A, WO2007/019806) creates an ac magnetic field with the flux lines 4 flowing substantially vertically out of a planar charging surface 5 (FIG. 2(a)). Since the lines of flux leave the charging surface vertically, the entire surface of the load in principle can be used to pick up the flux [S. C. Tang, S. Y. R. Hui and H. Chung, “Evaluation of the Shielding Effects on Printed-Circuit-Board Transformers using Ferrite Plates and Copper Sheets,” IEEE Transactions on Power Electronics, Vol. 17, No. 6, November 2002, pp. 1080-1088; S. C. Tang and S. Y. R. Hui, “Planar printed-circuit-board transformers with effective electromagnetic interference (EMI) shielding” U.S. Pat. No. 6,501,364; S. Y. R. Hui, “Apparatus and method of an inductive battery charger,” PCT patent application WO03/105308; S. Y. R. Hui and W. C. Ho, “A New Generation of Universal Contactless Battery Charging Platform for Portable Consumer Electronic Equipment,” IEEE Power Electronics Specialists Conference, 2004, Volume: 1 , 20-25 Jun. 2004, Pages: 638-644]. In practice, the area of the battery pack or the back cover of a portable electronic product can be used for the energy-receiving coil. This vertical flux approach makes it easier than the horizontal flux approach to design a slim energy-receiving module. Electro-magnetic shielding 6 is provided on the side of the charging surface opposite from the side on which a device to be charged is placed. This shielding prevents flux from being directed in the wrong direction (which would be a safety issue—especially if the battery charging platform was placed on a metal surface) and enhances the magnetic flux that is available for battery charging. Electromagnetic shielding is also added on the side of the energy-receiving coil opposite from the side to be placed on the charging surface as shown for example in FIG. 2(b). In such a battery charging platform a secondary winding is provided that is associated with a battery to be charged. The secondary winding picks up the magnetic flux and generates a charging voltage that is provided to the battery. Generally the secondary winding would be formed integrally with the battery such that a battery or a device containing the battery is placed on the charging surface with the secondary coil parallel to the surface such that it receives a maximum amount of magnetic flux. Alternatively, however, the secondary winding may be electrically connected to the battery but physically separate therefrom. In such a case the secondary winding may be formed as part of a secondary charging module that is placed on the charging surface. This possibility is particularly useful to allow the charging platform to be used with older electronic devices that are not otherwise designed for use with such a platform.
In both cases, the entire surface of the charging surface is energized for energy transfer. Although the concept of a “localized charging principle” has been disclosed previously in GB2389720A, U.S. Pat. No. 7,1642,55 and WO2007/019806, so far there is no systematic approach in designing an inductive battery charging pad that can meet the energy-efficiency, safety, electromagnetic compatibility requirements simultaneously. In “T. Sekitani, M. Takamiya, Y. Noguchi, S. Nakano, Y. Kato, K. Hizu, H. Kawaguchi, T. Sakurai, T. Someya, “A large-area flexible wireless power transmission sheet using printed plastic MEMS switches and organic field-effect transistors,” IEDM '06, International Electron Devices Meeting, December 2006, pp. 1-4,” a MEMS method has been proposed for inductive charging system, but such approach is restricted to relatively low-power and is very costly.